Prior art overcurrent protection devices, as shown in FIG. 6, are typically employed in a sealed compressor unit for a freezer or the like.
This device has a cylindrically-shaped casing 100 made of an insulating material with one open side of casing 100 which is placed directly against the housing of a compressor 124. First, second and third external connection terminals 102, 104 and 106 protrude from an opposite closed side 100a of casing 100. First and second external connection terminals 102 and 104 are integrally formed with a pair of fixed contacts 108 and 110 respectively provided within casing 100, and third external connection terminal 106 is connected to fixed contact 110 through a resistance heater 112 that is contained within casing 100.
At the center of the upper closed side 100a of the casing 100, a bolt 114 made of brass or the like is contained vertically in the casing extending both inside and outside of casing 100 and a disk-shaped bimetal 116 is attached by means of a rivet 115 or the like at the lower side of bolt 114 within casing 100. On the upper side of the bimetal 116, movable contacts 118 and 120 are welded at the locations which correspond to the fixed contacts 108 and 110 respectively.
At a time of normal operation, the bimetal 116 is located at a first position where the peripheral part of the disk bends upward with the center of the disk as the fulcrum so as to elastically compress the movable contacts 118 and 120 into contact with fixed contacts 108 and 110, thereby maintaining the switch circuit in a closed state. In this closed state, the electric current that has entered from the second or third external connection terminal 104 and 106 flows from the fixed contact 110 to the external connection terminal 102 through a movable contact 120, bimetal 116, movable contact 118 and the fixed contact 108.
FIG. 7 typically shows the construction of the above referred to overload protection device in an electric circuit. The first external connection terminal 102 is electrically connected to one terminal of an electric source 122; and a motor 126 in, for example, a compressor 124 is connected electrically between the other end of the electric source 122 and the third external connection terminal 106. In the case where it is not possible to provide resistance heater 112 inside casing 100, a terminal 126b of the motor 126 is electrically connected to second external connection 104.
When the switch is in a closed state, the electric current that flows to the motor 126 also flows to the bimetal 116 and heating resistor 112, with the bimetal 116 being heated by resistance heating caused by the current flowing through it and also by the heat from resistance heater 112. In addition, the bimetal 116 is also heated by the radiant heat from the compressor 124; however, the extent of this radiant heating is small as compared with the heating caused by resistance heating.
The ordinary case where motor 126 of compressor 124 requires protection is the case where the electric current has exceeded a certain rated value due to an overload or locked rotor state. In such a case, typically the cooling ability of a condenser (which is not shown in the drawing) is reduced and therefore the amount of the work on the compressor 124; and thus, the load on the motor 126 becomes excessive. This condition results in a current overflow and the possibility of damage to motor coils. Also, in the case where the operation of the compressor 124 is started again immediately after its stoppage, there is a possibility that the piston is not able to compress the coolant gas if there is a stagnant coolant gas at high temperature and under high pressure on the output side. This condition also causes the motor to demand abnormally high current.
When the electric current that flows to the motor 126 increases as described above, there is an increase in the heat due to resistance heating within the bimetal 116 with the result that the temperature of the bimetal 116 rises. When it rises to a prescribed first action temperature such as 160 degrees centigrade for example, the bimetal 116 snaps over center thereby being displaced to the second position where the peripheral part of the disk bends downward as is shown by the dotted line 116' in FIGS. 6 and 7. In this position, the movable contacts 118 and 120 that are fixed to the top of the bimetal 116 become separated from the fixed contacts 108 and 110 respectively with a result that the switch circuit is opened and the electric current is shut off. Due to this current shut-off, the possible damage to the coils of the motor 126 is prevented.
When the electric current is shut off, the heating within the bimetal 116 stops. When the bimetal 116 is cooled to the prescribed second action temperature such as, for instance, 80 degrees centigrade, the bimetal 116 snaps and moves from the second position back to the first position thereby closing the switch circuit. Due to this movement of the bimetal, the electric current once again flows and the operation of the compressor 124 is re-started.
Certain problems may occur with these prior art protectors. In operation, the bimetal 116 may gradually wear out as it snaps repeatedly between the first position and the second position. If a crack develops in a bimetal, it typically will no longer snap as desired, even if it is heated to a temperature above the action temperature with the result that the movable contacts 118 and 120 do not move out of contact with fixed contacts 108 and 110. Also, there are cases where even if the bimetal 116 attempts to snap regularly, the movable contacts 118 and 120 become "welded" to the fixed contacts 108 and 110 and are not separated from them. In such cases, there is a need for cutting of the electric current. However, the overload protection device of the prior art, as described above, does not have means for doing so with the result that the electric current continues flowing and that the motor 126 can be damaged due to an overload.
In order to cope with this problem, it has been the case in the past to install a separate thermostat on the compressor with the switch circuit of this thermostat being connected in series with the switch circuit of an overload protection device.
This solution, of course, involves additional cost and handling and installation problems. Further, an overload protection device of the past only responds effectively to an overcurrent, but does not also adequately protect against excessive rise in the temperature of the load. That is, the temperature of the load (compressor 124) is transmitted to the bimetal 116 only in the form of radiant heat through the open lower surface of the casing 100 with the result that the rate of the response to such an excessive rise in the temperature of the load has been slow.
Still further, in the conventional overload protection device, the bimetal 116 typically becomes cooled and returns to the first position before the temperature of the load (compressor 124) has been sufficiently lowered subsequent to a shut-off of the electric current. This can cause insufficient protection of the load or an increase in the number of the actions of the switch thereby shortening the life of the contact mechanism.
Lastly, the overload protection device according to prior art has lacked the easy freedom of adjustment of the values of electric current shut off or overload current protection.